Lithium ion batteries are being intensively pursued for hybrid electric vehicle (HEV) and plug-in hybrid electric vehicle (PHEV) applications. Both the 4 V spinel LiMn2O4 and 3.4 V olivine LiFePO4 cathodes have drawn much attention in this regard because Mn and Fe are inexpensive and environmentally benign. Additionally, these cathodes provide a higher rate capability and better safety compared to layered oxide cathodes. However, both LiMn2O4 and LiFePO4 cathodes have limited energy density due to their low capacity or operating voltage. One way to improve the energy and power density is to increase the operating voltage. In this regard, the 5 V spinel cathode LiMn1.5Ni0.5O4 has drawn much attention due to a nearly flat operating voltage close to 5 V and an acceptable capacity arising from operation of the Ni2+/3+ and Ni3+/4+ redox couples.
The LiMn1.5Ni0.5O4 cathode, however, can be characterized by suboptimal cycling performance in a conventional carbonate electrolyte, and this may be due to the large lattice strain during cycling, which involves the formation of three cubic phases with a large lattice parameter difference during the charge-discharge process. Other contributors to suboptimal cycling performance can include the LixNi1-xO impurity, and the corrosion reaction between the cathode surface and the carbonate electrolyte at the high operating voltage of approximately 5 V.
Partial substitution of Mn and Ni in LiMn1.5Ni0.5O4 by other elements such as Li, Al, Mg, Ti, Cr, Fe, Co, Cu, Zn and Mo has been pursued to improve the cyclability, as discussed in U.S. Pat. No. 6,337,158 (Nakajima); and in Liu et al, J. Phys. Chem. C 13:15073-15079, 2009. Although improvement in cycling performance can be achieved in a conventional carbonate electrolyte at room temperature by partial cation substitution, high-temperature cycling performance still remains a problem due to the intrinsic instability of the traditional carbonate electrolyte and the accelerated decomposition reaction at elevated temperature.
U.S. Patent Application Publication No. 2010/0035162 (Chiga) described a nonaqueous electrolyte for use in a secondary battery that comprises a chain fluorinated carboxylic acid ester represented by the formula CH3COOCH2CH3-xFx, wherein x is 2 or 3, and a film-forming chemical that decomposes in the range of +1.0 to 3.0 V based on the equilibrium potential between metal lithium and lithium ion. This electrolyte was in various embodiments used in a secondary battery that was provided with a lithium-transition metal oxide cathode having a charge cut-off voltage of 4.2 V.
Despite the efforts in the art as described above, a need remains for a lithium ion battery that operates at high voltage (i.e. up to about 5 V) and has improved cycling performance at high temperature.